1. Field
The disclosed concept pertains generally to circuit interrupters and, more particularly, to arc fault circuit interrupters. The disclosed concept also pertains to methods of detecting arc faults.
2. Background Information
Circuit breakers are used to protect electrical circuitry from damage due to an overcurrent condition, such as an overload condition or a relatively high level short circuit or fault condition. In small circuit breakers, commonly referred to as miniature circuit breakers, used for residential and light commercial applications, such protection is typically provided by a thermal-magnetic trip device. This trip device includes a bimetal, which heats and bends in response to a persistent overcurrent condition. The bimetal, in turn, unlatches a spring powered operating mechanism, which opens the separable contacts of the circuit breaker to interrupt current flow in the protected power system.
An arc fault circuit interrupter (AFCI) is a device intended to mitigate the effects of arc faults by functioning to deenergize an electrical circuit when an arc fault is detected. Non-limiting examples of AFCIs include: (1) arc fault circuit breakers; (2) branch/feeder arc fault circuit interrupters, which are intended to be installed at the origin of a branch circuit or feeder, such as a panelboard, and which may provide protection from series arc faults, ground faults and line-to-neutral faults up to the outlet; (3) outlet circuit arc fault circuit interrupters, which are intended to be installed at a branch circuit outlet, such as an outlet box, in order to provide protection of cord sets and power-supply cords connected to it (when provided with receptacle outlets) against the unwanted effects of arcing, and which may provide protection from series arc faults, line-to-ground faults and line-to-neutral faults; (4) cord arc fault circuit interrupters, which are intended to be connected to a receptacle outlet, in order to provide protection to an integral or separate power supply cord; (5) combination arc fault circuit interrupters, which function as either a branch/feeder or an outlet circuit AFCI; and (6) portable arc fault circuit interrupters, which are intended to be connected to a receptacle outlet and provided with one or more outlets.
During sporadic arc fault conditions, the overload capability of a conventional circuit breaker will not function since the root-mean-squared (RMS) value of the fault current is too small to activate the automatic magnetic trip circuit. The addition of electronic arc fault sensing to a circuit breaker can add one of the elements needed for sputtering arc fault protection-ideally, the output of an electronic arc fault sensing circuit directly trips and, thus, opens the circuit breaker. See, for example, U.S. Pat. Nos. 6,710,688; 6,542,056; 6,522,509; 6,522,228; 5,691,869; and 5,224,006.
Arc faults can be series or parallel. Examples of a series arc are a broken wire where the ends of the broken wire are close enough to cause arcing, or a relatively poor electrical connection. Parallel arcs occur between conductors of different potential including, for example, a power conductor and a ground. Unlike a parallel arc fault, series arc faults do not usually create an increase in current since the fault is in series with the load. In fact, a series arc fault may result in a slight reduction in load current and not be detected by the normal overload and overcurrent protection of conventional protection devices. Even the parallel arc, which can draw current in excess of normal rated current in a circuit, produces currents which can be sporadic enough to yield RMS values less than that needed to produce a thermal trip, or at least delay operation. Effects of the arc voltage and line impedance often prevent the parallel arc from reaching current levels sufficient to actuate the instantaneous trip function.
Both safe and unsafe series arcs occur in power circuits, such as electrical power distribution systems (or electrical distribution systems). One example of safe series arcs occurs in the commutator brushes of direct current (DC) and universal motors. In order to minimize brush heating and erosion, universal motors are designed to minimize the net duration and thereby the total energy dissipated by commutation arcing. The materials and physical shape of the commutator brushes are chosen, in order that they are minimally affected by the arcing. Hence, the series arcs that occur in universal motors are intended and perform a constructive purpose.
In contrast, a combination of random processes is known to produce an unintended series arc in electrical distribution systems. The unintended series arcs may become hazardous when the net duration and total energy dissipation of the arc are uncontrolled, and localized heating produced by the arc may damage or even ignite things nearby. Thus, unsafe series arcs in electrical distribution systems could theoretically result in loss of property or even life.
Arcs that safely occur within universal motors and unsafe series arcs that happen by random chance are the exact same physical phenomena; however, one occurs by design and creates a benefit while the other is unintended and may be destructive. Thus, there is a need for circuit interrupters that accurately distinguish between safe series arcing (e.g., without limitation, in universal motors) and unintended, potentially hazardous series arcs, and trip in response to only the latter. This need is further compounded by the reality that, of all series arcs in power distribution systems, only a fraction are the unintended, unsafe variety.
There is room for improvement in arc fault circuit interrupters.
There is also room for improvement in methods of detecting arc faults.